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This application discloses an innovative fiber optic sensor technology, and the associated electro-optic subsystems and signal-processing algorithm.
Interferometric Sensor Principle of Operation
Shown in FIG. 1 are five examples of potential sensor configurations that can be used to monitor displacement using the disclosed electro-optic configuration and algorithm. Each example given utilizes the sensor (fiber) end face/gap interface to generate a first surface reflection. With each configuration shown, two sensing schemes are available. The first scheme is a fixed fiber/mobile reflector methodology and the second is a mobile fiber/fixed reflector methodology. With respect to dilatometry, a fixed fiber/mobile reflector scheme is employed.
The first configuration shown in FIG. 1 (a) places a graded-index (GRIN) lens at the end of a fiber. This configuration has the advantage of being able to collimate the light beam to enable longer-distance measurements (larger dynamic range). Configuration (b) allows for a decrease in the cross-sectional sensor profile of configuration (a) through the implementation of a spliced multi-mode graded-index fiber. Note that with this configuration the length of the multi-mode fiber is not arbitrary. Configuration (c) further reduces the cross-sectional sensor profile. Configuration (d) represents the configuration implemented within this disclosure. A standard single-mode fiber is cleaved and/or polished at an angle xcfx86, which has been shown to improve the signal to noise ratio. Finally, configuration (e) represents an alternative approach.
In all five configurations the operation is the same: light energy enters from the left in the lead-in fiber and approximately 4% is reflected at the glass/gap interface (this is termed the Fresnel reflection). The remaining energy is coupled into the gap region, where it traverses the gap and is reflected by a reflector. After traveling back to the fiber interface some of the reflected energy is coupled back into the fiber, where it modulates the Fresnel reflection. If the reflecting modulation is in phase with the Fresnel wave the two constructively interfere, else if they are out of phase they destructively interfere. The first surface reflection can be strengthened or weakened through the use of specific coatings (see FIG. 3).
Sensor Fabrication
Fringe visibility, or the absolute height difference between the maximum and minimum peaks in the interferogram, is an important parameter that directly impacts system accuracy. In order to maximize fringe visibility, hence, increase signal to noise ratio, several options are available: 1) the use of GRIN lenses, 2) the use of a short piece of graded index fiber fusion spliced at the sensor tip, or 3) the use of polishing to increase the reflectance of the two surfaces. Additionally, recent calculations by Airak personnel indicate that a slight angle, induced at the fiber end-face, can contribute to a slight increase in fringe visibility (see FIG. 1).
Optical System Configuration
For this disclosure the configuration of the optical subsystems is as follows: a light emitting diode (LED) with a center wavelength of 850 nm and 20 mn full width half-maximum serves as the light source and a StellarNet, Inc. EPP2000 spectrometer serves to digitize the broadband interferometric signal. The spectrometer possesses a 2048 pixel array and a wavelength range of 550-1000 nm. Interface to the computer is through an EPP parallel port. There are many optical system configurations possible and configuration examples addressed are not meant to limit in any manner the number of realizable configurations. The novel aspects of the system are 1) the use of a spectrometer, 2) the sensor configurations and 3) the signal processing algorithm.
In order to validate the statement aforementioned concerning the number of available optical configurations for measuring displacements with high accuracy over a given dynamic range (i.e. specific to the application of interest), computer simulations are given below. The simulations rely on white-light interference patterns with known parameters [displacement gap and signal-to-noise ratio (SNR)]. The variables of interest are the mean and standard error of the source profile, 2) the number of pixels in the spectrometer, and 3) the wavelength range of the spectrometer.
FIG. 4 through FIG. 7 depict the results of the computer simulations for two different optical configurations. The mean error (bias error) represents the accuracy of the measurement and can be reduced through proper calibration. The random error (variance) represents the resolution of the system and cannot be reduced through calibration1.
1 J. S. Bendat and A. G. Piersol, Random Data, John Wiley and Sons, 1986. 
FIG. 4 and FIG. 5 are representative of the first configuration. This configuration employs a LED centered at 850 nm with a 50 nm full-width half-max and a spectrometer with a wavelength range of 750-950 nm. Typically, the SNR realized for the disclosed system was between 30 and 40 dB, therefore the mean error was less than 0.5 nm and the resolution was less than a nanometer.
FIG. 6 and FIG. 7 are representative of the second configuration. This configuration employs a broadband white-light source and uses StellarNet""s EPP2000c spectrometer which has a wavelength range of 200-850 nm. This results in a system accuracy increase of approximately two orders of magnitude.
These computer simulations indicate how the system accuracy, range and resolution are affected by changing the optical parameters. Using this information, the optical system can be tailored to meet specific requirements in range or accuracy.
Although several options exist for the design and construction of the integrated opto-electronics, common to all systems are the LED light source, the LED driver and optical coupler subsystem, the sensors, at least one spectrometer, some form of analog to digital conversion subsystem, a control and feedback subsystem, a digital to analog conversion subsystem, and the signal processing algorithm to process the signal.